With such treatment it may be necessary to use three separate catheters. With a first catheter, provided with a dilatation balloon, the narrowing (or stenosis) is dilated. With a second catheter a stent is subsequently positioned at the site of the narrowing. This is done by positioning a stent in compressed state, placed around the balloon member of a catheter intended for that purpose in the area of the narrowing, inflating the balloon as a result of which the stent will expand, and generally, plastically deform, and subsequently deflating and withdrawing the balloon. A third catheter comprises an ultrasonic probe, and is used to generate an image of the narrowing or of the stent in situ by recording the transmitted ultrasonic waves on a suitable receiver and converting them into an image.